All About Circuits Forum How is it that there is continuity and a voltage reading yet no short circuit?
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#1
02-05-2009, 03:14 AM
 foolios Member Join Date: Feb 2009 Location: U.S. Posts: 97
How is it that there is continuity and a voltage reading yet no short circuit?

Excuse me if I don't explain this correctly. I am trying to get my head around how this works. Please ask for clarifications and I will do my best to do so.

My question is about how is it that when I do a voltage check from line side to load side at a meter box with a main breaker switch closed that I will get 120V and that I will get a beep when doing a continuity test yet there isn't a short in the system to allow a complete circuit?

If there is nothing plugged into the walls of a residence, how is it that there is a circuit for the voltage reading and continuity test to occur?

See my diagram below. At the wall outlet, there is an open break in the circuit until a load is plugged in. How then am I getting these results when I do my test before the main breaker when nothing is plugged into the wall outlets in the entire residence?
I was told it was because of the ground wire at the outlets. But that makes me think that that would actually be a short circuit if that was the case and we'd have continuous electrical usage...

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Thanks in advance for any explanations.

Last edited by foolios; 01-23-2010 at 02:15 PM.
#2
02-05-2009, 04:14 AM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

A breaker is not "opened or closed". It is turned on or it is turned off.
If the breaker is turned on then of course voltage is at the wall outlet even if nothing is plugged in to draw current.

A continuity tester tests wires that are not powered, the breaker must be turned off.
#3
02-05-2009, 04:26 AM
 SgtWookie Expert Member Join Date: Jul 2007 Location: In the vast midwest of the USA; CST Posts: 22,031

Ground is connected to Neutral at the main breaker panel. In most normal circumstances, this is the only place that the two meet.

Outside the residence, usually right near the electrical service entrance, there are one or two 8' long copper-clad stakes driven into the ground, with an AWG 4 or larger conductor clamped to it. This usually gives a reliable earth ground for many years.

This conductor is run to the main service panel, and attached to the ground buss. There is a separate Neutral buss, however the two are tied together. This keeps the line voltages from rising to very high levels, should a fault occur in the power company's transformer; the transformer's fuses will blow instead of creating a very dangerous situation.

Although Ground is connected to Neutral at the service panel, the ground wire should never be used to carry return current to the service panel. The ground wire is there to provide safety for the human users of the electrical service; the chassis of appliances are connected to the ground wires to protect them from an internal fault.

GFCI (Ground Fault Circuit Interrupt) breakers such as used in kitchen, bathroom and outdoor recepticles detect current flow in the ground circuit, and will open the circuit (trip the breaker) if the current in the ground wire exceeds around 15mA.
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#4
02-05-2009, 09:51 AM
 studiot E-book Developer Join Date: Nov 2007 Location: Somerset UK Posts: 3,957

Quote:
 how is it that there is a circuit for the voltage reading
Please remember that you need a circuit for current to flow, but not for voltage to be present.

Think about an ordinary battery there is always voltage between the terminals (for a good one), whether it is connected into a circuit or not.
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#5
02-05-2009, 11:17 AM
 mik3 Senior Member Join Date: Feb 2008 Location: Cyprus, but now in UK (GMT+0) Posts: 4,846 Blog Entries: 9

Where do you place the meter probes when you perform the voltage measurement and continuity test?
Is the main braker turned off during tests?
#6
02-05-2009, 11:40 PM
 foolios Member Join Date: Feb 2009 Location: U.S. Posts: 97

Quote:
 Originally Posted by mik3 Where do you place the meter probes when you perform the voltage measurement and continuity test? Is the main braker turned off during tests?
I am placing the meter probe from the line side of the meter box(the jaws in it) to the load side of the meter box.

What I test for is to make sure that the main circuitbreaker is off(open).
What I want to understand is:
If the main switch is closed(on) and there is nothing plugged into any outlet in the residence and there is nothing on that would draw current, would I get a voltage reading? If so, why?
How can there be a voltage reading if there is nothing plugged in or on to be powered up/complete the circuit if that's so?

Also, in this same scenario, would a continuity test be possible? As I read above, continuity tests can only be done with the power off, so in this case of a main closed, would that mean a continuity test can't be done even though there is nothing plugged in to complete the circuit?

THanks for the replies.
#7
02-05-2009, 11:50 PM
 mik3 Senior Member Join Date: Feb 2008 Location: Cyprus, but now in UK (GMT+0) Posts: 4,846 Blog Entries: 9

If the main breaker is turned on there will be a voltage between neutral and live wires on the outlet. The voltage on the outlet does not depend on the appliance connected to it but it always there and waits to power an appliance. When an appliance is connected it will draw some current and this current depends on the power of the device. Don't confuse voltage with current.

As for the continuity test don't make it with power on, you may get shocked or blow the meter.
#8
02-05-2009, 11:58 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

If you are looking for continuity under the stated conditions -
Quote:
 My question is about how is it that when I do a voltage check from line side to load side at a meter box with a main breaker switch closed that I will get 120V and that I will get a beep when doing a continuity test yet there isn't a short in the system to allow a complete circuit?
then you have to read continuity. Having the breaker closed means it is acting like a switch that is turned on. Checking from one side of it to the other will naturally show continuity.

One verifies the state of a breaker by looking for voltage between the hot lead and its associated neutral. If you still see 120 VAC, the breaker is closed. If open, the voltage will not be present.

Checking continuity is done on circuits which have been disconnected from the power source. If you suspect an open, the check may be done with a voltmeter and power on. You place a lead at the source and probe with the other. Finding a spot where the source voltage is read means there is an open between the metered points.
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#9
02-06-2009, 12:41 AM
 foolios Member Join Date: Feb 2009 Location: U.S. Posts: 97

I need to break this down a little. Lemme isolate just the voltage concerns.
When I test for voltage at the meter box and the breaker is open, I get no reading. Now, isn't there potential for voltage right here at the box just like if the breaker was closed and there was nothing plugged into the wall outlet; which makes this the same test when I'm also testing for voltage at the wall outlet? There is a break isn't there? Whether it's at the open main or whether it's at the open wall outlet. I don't understand how it is any different between the two when it comes to reading potential voltage.

As far as if it's about completing a circuit, of which is where I believe my misunderstanding lies.
I can see if there was an appliance that was plugged in and on. This completes a circuit. I should get a voltage reading. Voltage wants to push current.
But how is this occuring when there isn't an appliance plugged in and on to complete the circuit? Yes, there is a neutral connected to each outlet but it's not directly connected, otherwise there would be a short.

Sorry if I'm beating a dead horse, I really am curious to understand at least this part of it.
#10
02-06-2009, 12:54 AM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

The situation is very like an earlier example, where you can see voltage across a battery in the absence of any circuit than the meter. That is like reading 120 VAC at an outlet that is otherwise open. The voltage is just the difference in potential between the hot wire and the neutral.

If you open the breaker feeding that branch, the voltage goes away at the outlet, and at every other part of that branch.

If I understand this -
Quote:
 Now, isn't there potential for voltage right here at the box just like if the breaker was closed and there was nothing plugged into the wall outlet; which makes this the same test when I'm also testing for voltage at the wall outlet? There is a break isn't there? Whether it's at the open main or whether it's at the open wall outlet. I don't understand how it is any different between the two when it comes to reading potential voltage.
- you have read across an open breaker and see voltage. With a high impedance voltmeter and a bit of leakage in the branch, that's not too surprising. Plug in any load, though, and that voltage will go away.
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 Tags circuit, continuity, reading, short, voltage

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